NEW YORK (REUTERS, AFP) - There is no guarantee the National Football League (NFL) will complete its season due to the Covid-19 pandemic, although some fans may be allowed to attend games if things go well, said Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases.

The NFL is to kick off on Sept 10 and unveiled its 2020 schedule last week, with the expectation of playing games with fans in the stadiums.

"The virus will make the decision for us," Fauci told NBC's Peter King over the weekend. "I think it's feasible that negative testing players could play to an empty stadium.

"If the virus is so low that even in the general community the risk is low, I could see filling a third of the stadium or half the stadium so people could be six feet apart. It's going to depend."

However, he also warned that he was against staging a season if the virus was as widespread in September as it is now.

"Right now, if you fast forward, and it's now September, the season starts, I say you can't have a season - it's impossible," Fauci said. "There's too much infection out there."

The key, he added, would be whether there was enough capacity for the 32 NFL teams to test players, coaches and other personnel at least twice a week once the season began.

"Right now, it would be overwhelmingly piggish. But by the end of August, we should have in place antigen testings... You could test millions of people," he said, referring to the new category of rapid tests which the US Food and Drug Administration approved on May 9.

"If you really want to be absolutely certain, you'd test all the players before the game... To be 100 per cent sure, you've got to test every day. But that's not practical and that's never going to happen.

"But you can diminish dramatically by testing everybody Saturday night, Sunday morning, and say 'OK, only negative players play.'"

But he warned that if four players on a 53-man NFL roster tested positive on the eve of a Sunday game, it might hint that more cases are present among those still testing negative.

"It's likely that if four of them are positive and they've been hanging around together, that the other ones that are negative are really positive," Fauci said.

No player testing positive for the virus could be allowed to play, he said, adding: "It would be malpractice in medicine to put him on the field, absolutely."

That is because the NFL's close contact would be an easy environment in which to spread the virus.

"If people are in such close contact as football players are on every single play, then that's the perfect set up for spreading," Fauci said. "As soon as (an infected player) hit the next guy, the chances are that they will be shedding virus all over that person."

Fauci said it was inevitable the virus would return as the weather got cooler, and that it could set up a "bad fall and a bad winter," adding: "There's no chance we're going to be virus-free."

"As for the football season, it will be entirely dependent on the effectiveness with which we as a society respond to the inevitable outbreak that will occur," he added.

"Even if the virus goes down dramatically in June, July and August, as the virus starts returning in the fall, it would be in my mind, shame on us if we don't have in place all of the mechanisms to prevent it from blowing up again.

"So the ability or not to actually have a football season is... unpredictable depending upon how we respond in the fall. You'll have to play it by ear according to the level of infection in the community."